In general, a fingerprint constituted with a large number of stripe-pattern ridgelines has two significant features such as being unchanged through one's life and being different for every person. Thus, it has been utilized as a means for crime investigations from the old times. In particular, collation using a latent fingerprint remained in a crime scene is an effective means for investigations. A fingerprint matching system utilizing a computer is employed in many police organizations to conduct latent fingerprint matching.
However, many of the latent fingerprint images are of low quality and contain a noise, so that examinations thereof by experts are difficult to be done. Further, it is a major factor of an obstruction for achieving automation. There are various kinds in the noise of the latent fingerprint. Among those, there is a dot pattern noise in which dots are lined in a grating form at an equal interval.
As an example of the case where the dots are lined in a grating form at an equal interval, there is a case where a latent fingerprint remains on the dots of a check or a picture area of newspaper, for example.
A set of such dots is printed in order to increase the density of the picture area, and those dots are disposed in a grating form at an equal interval (here, this is called a dot pattern; a dot pattern may sometimes be called a screentone). Hereinafter, the noise constituted with a dot pattern is called as a dot pattern noise.
As shown in FIG. 4, there is a case where fingerprint ridgelines of a latent fingerprint remain on a print part of a check and a dot pattern noise appears noticeably. In such case, the dot pattern noise becomes a factor of obstruction, thereby making it difficult to extract the fingerprint ridgelines accurately.
Further, as a related technique for removing the pattern noise having periodicity, it is common to employ the Fourier transformation. This technique is disclosed in Non-Patent Document 1, for example.
FIG. 15A to FIG. 15F show examples of a case where a dot pattern noise is removed by using the method of Non-Patent Document 1.
Note here that FIG. 15A, FIG. 15D are input images, FIG. 15B, FIG. 15E are power spectral images on a Fourier transformation plane, and FIG. 15C, FIG. 15F are images showing a case where periodic background noise is removed by reducing the peak components of the power spectral image and inversely transforming the result thereof by the method depicted in Non-Patent Document 1.
In the power spectral images shown in FIG. 15B and FIG. 15E, coordinates closer from the origin show lower frequency components, while coordinates farther form the origin show higher frequency components. Further, the density of the pixels shows the intensity (power) of the frequency component at that coordinate.
Further, there is an entirely uniform dot pattern noise exists in the input image shown in FIG. 15A. Thus, in the power spectral image shown in FIG. 15B, the intensity of the frequency components of the dot pattern noise in the power spectrum becomes extremely large.
With the method depicted in Non-Patent Document 1, a large-intensity component is supposed as a frequency component of a periodic noise and it is decreased. As a result, in the background noise removal result shown in FIG. 15C, it is shown that the noise components are removed effectively.
However, as the input image of FIG. 15D, when there is the dot pattern noise of a different feature existing in the processing-target area of a latent fingerprint, the intensity of the noise components on the power spectrum of FIG. 15E becomes weak. That is, compared to FIG. 15B, the peak components of the intensity of the dot pattern noise are dispersed. As a result, removal of the noise components in the background noise removal result shown in FIG. 15F becomes insufficient.
Further, FIG. 16A and FIG. 16B show a case where a printed character is contained in the processing target area. FIG. 16A is an input image, and FIG. 16B shows an image from which the periodic background noise is removed by the method depicted in Non-Patent Document 1.
According to this, it can be seen that a new noise pattern that is not present in the input image is generated in the printed character part of FIG. 16B due to a side effect of removing the periodic background noise, and that the quality for discriminating the printed character part and a fingerprint is deteriorated because of the generated noise pattern.
Further, as a related fingerprint ridgeline enhancing method, there are disclosed various methods which extract the direction and periodicity of a local ridgeline and enhance it by filtering processing that matches the extracted direction and periodicity (Patent Document 1, Non-Patent Document 2).    Patent Document 1: Japanese Unexamined Patent Publication 2002-99912    Patent Document 2: Japanese Unexamined Patent Publication 2001-160903    Patent Document 3: Japanese Unexamined Patent Publication 2007-312049    Patent Document 4: Japanese Unexamined Patent Publication Hei 11-078183    Non-Patent Document 1: Canon, et al., “Background Pattern Removal by Power Spectral Filtering”, Applied Optics, Mar. 15, 1983    Non-Patent Document 2: Hong, et al., “Fingerprint Image Enhancement: Algorithm and Performance Evaluation (1998)”, IEEE Transactions on Pattern Analysis and Machine Intelligence, 1998
However, in a case where the related technique disclosed in Non-Patent Document 1 is applied for removing a dot pattern noise of a latent fingerprint image, it is necessary for the dot pattern noise to appear uniformly in a wide range, and the effect thereof becomes limited.
Further, in a case where dot pattern noises of different features (properties) exist adjacently, the concentration degree of the dot pattern noise components on a Fourier transformation plane becomes weak (coarse). Thus, extraction of the noise components becomes difficult, which results in limiting the noise removal effect as well.
Further, there is also such an inconvenience that a side effect such as giving a bad influence on the fingerprint ridgelines is generated in an area without the dot pattern noise, which obstructs extraction of the fingerprint ridgelines.
Further, regarding the related techniques of Patent Document 1 and Non-Patent Document 2, enhancement of the fingerprint ridgeline is not effective in a case where the direction and the periodicity of the fingerprint ridgelines cannot be extracted accurately due to an influence of the dot pattern noise. Thus, the fingerprint ridgelines cannot be enhanced in a fine manner.
Further, Patent Documents 2 to 4 limit the existence of the noise and the area thereof, and perform removal of the noise that matches the feature of the noise by limiting it to the noise area. Those techniques do not intend to avoid a bad influence imposed upon the other areas than the noise area.
An object of the present invention is to provide a pattern noise removal device, a pattern noise removal method, and a pattern noise removal program, which effectively remove a periodic background pattern noise such as a dot pattern from a fingerprint image.